Move MSRV to cargo.toml and remove build.rs (#143)

* Move MSRV to cargo.toml and remove build.rs

This reworks MSRV detection logic, using the `rust-version` field
in Cargo manifest. It allows dropping `build.rs` and removing
the `version_check` build-dependency.

Ref: https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field

* touching a file to hopefully re-trigger checks

---------

Co-authored-by: Elliott Mahler <[email protected]>

Author: lucab

Cargo.toml

@@ -4,6 +4,7 @@ name = "rustfft"
 version = "6.3.0"
 authors = ["Allen Welkie <allen.welkie at gmail>", "Elliott Mahler <join.together at gmail>"]
 edition = "2018"
+rust-version = "1.61"
 
 description = "High-performance FFT library written in pure Rust."
 documentation = "https://docs.rs/rustfft/"
@@ -58,5 +59,3 @@ libc = "=0.2.163"
 # it probably isn't wise to stay on an old version for a long time, but we'll have to upgrade MSRV to upgrade
 once_cell = "=1.20.2"
 
-[build-dependencies]
-version_check = "0.9"